The invention pertains to a process for the quenching of workpieces with gases in a heat-treatment system and for recooling the circulated gases on cooling surfaces in at least one heat exchanger.
High-quality tools of hot-forming and cold-forming tool steels and of high-performance high-speed steels are usually heat-treated today in vacuum heat-treatment systems by high-pressure gas quenching.
As a result of the further development of high-pressure gas quenching in the direction of higher gas pressures and gas velocities, and also because of the choice of suitable quenching gases, it is possible to expand the use of this technology to the area of low-alloy steels and case-hardening steels. In these cases, work is carried out at gas pressures of up to 20 bars. The current system technology, however, makes it possible only to treat workpieces with relatively thin walls or cross sections and small batch sizes.
This technology leads necessarily to the use of high-pressure tanks for the heat treatment and gas quenching and of heat-treatment systems with thick walls. The construction and sealing of the flange joints as well as of the doors or covers of the heat-treatment systems are especially challenging.
The level of the quenching intensity which can be achieved is largely determined by the choice of gas, the gas pressure, the gas velocity, and the gas temperature. The level of the gas temperature influences the amount of heat which can be carried away from the batch and thus affects the quenching intensity by way of the thermal conductivity coefficient .alpha. and the driving temperature difference between the batch and the quenching gas.
The level of the gas temperature is influenced by, among other things, the heat exchanger used to recool the quenching gas. Through the use of cooling water as coolant on the secondary side of the heat exchanger, the level of the gas outlet temperature behind the heat exchanger remains limited even at optimum efficiency to values on the order of about 30.degree.-50.degree. C.